Need illumination wire hook up for tach????

I tried hooking it to the light bulbs in ac heater controls. I spliced into gray wire, but it did not illuminate. There was also a white wire I believe going to the bulbs. Which one should work or which wire in rear of stereo is for illumination (that would be easier). Thanks

i was looking for that wire to hook my head unit into and it was a brown one.

Ok thanks, brown wire on stereo.